The correct answer is $\boxed{\frac{1}{{20}}^{{\text{th}}}}$ of span.
The width of a beam is an important factor in its design. The width must be sufficient to support the load that the beam will be carrying, but it should not be so wide that it becomes unnecessarily heavy. A good rule of thumb for the initial estimate of the width of a beam is to assume that it is $\frac{1}{{20}}^{{\text{th}}}$ of the span. This will ensure that the beam is strong enough to support the load, but it will not be so wide that it becomes unnecessarily heavy.
Option A, $\frac{1}{{15}}^{{\text{th}}}$ of span, is too narrow. A beam that is this narrow would not be strong enough to support the load.
Option B, $\frac{1}{{20}}^{{\text{th}}}$ of span, is a good rule of thumb for the initial estimate of the width of a beam.
Option C, $\frac{1}{{25}}^{{\text{th}}}$ of span, is too wide. A beam that is this wide would be unnecessarily heavy.
Option D, $\frac{1}{{30}}^{{\text{th}}}$ of span, is too wide. A beam that is this wide would be unnecessarily heavy.